gkeenan - Arby's site has the nutritional information, but that info includes the bun. I wrote to them and asked for carb counts of the roast beef only, and here's what they told me:

0.09 grams per ounce, a regular roast beef has 3 ounces, larger sandwiches have 4 ounces!

edited to add: Yes, all of the fast food restaurants have nutritional information, but not all of them gives specific details on the meat only. Some sites, like Burger King and McDonalds allows you to remove items such as buns and ketchup! If you find a site that doesn't have that feature, e-mail them and ask. Arby's wrote me back within 2 days!